And after a quick search of MR you can find a free, drag and drop tool for Macs to properly create an epub from a folder that holds an epub's contents.
Here is the link:
https://www.mobileread.com/forums/sh...49&postcount=1
Download and install the ePubZip/Unzip tool then drag or drop the folder with the .epub extension inside iBooks onto it, and it shoudl create a real .epub file you should be able to load in Calibre.